Holographic projector

ABSTRACT

There is provided a holographic projector comprising a processing engine, spatial light modulator ( 403 B), light source ( 401 B) and light-receiving surface ( 405 B). The processing engine outputs a computer-generated diffractive pattern defining a propagation distance to an image plane. The spatial light modulator displays the computer-generated diffractive pattern. The light source illuminates the spatial light modulator at an angle of incidence (theta) greater than zero. The light-receiving surface receives spatially-modulated light from the spatial light modulator. The light-receiving surface is substantially parallel to the spatial light modulator (alpha-theta). The light-receiving surface is separated from the spatial light modulator by the propagation distance defined by the computer-generated diffractive pattern.

CROSS-REFERENCE TO RELATED APPLICATIONS

This application is a U.S. National Stage application of InternationalPatent Application no. PCT/EP2019/065082, filed Jun. 10, 2019, whichclaims the benefit of priority of United Kingdom Patent Application no.1809983.8, filed Jun. 18, 2018.

FIELD

The present disclosure relates to a projector. More specifically, thepresent disclosure relates to a holographic projector and holographicprojection system. Some embodiments relate to a head-up display and ahead-mounted display. Some embodiments relate a method of reducing thesize of the image spots in a holographic replay field and someembodiments relates to a method of increasing the resolution in aholographic replay field.

BACKGROUND AND INTRODUCTION

Light scattered from an object contains both amplitude and phaseinformation. This amplitude and phase information can be captured on,for example, a photosensitive plate by well-known interferencetechniques to form a holographic recording, or “hologram”, comprisinginterference fringes. The hologram may be reconstructed by illuminationwith suitable light to form a two-dimensional or three-dimensionalholographic reconstruction, or replay image, representative of theoriginal object.

Computer-generated holography may numerically simulate the interferenceprocess. A computer-generated hologram, “CGH”, may be calculated by atechnique based on a mathematical transformation such as a Fresnel orFourier transform. These types of holograms may be referred to asFresnel or Fourier holograms. A Fourier hologram may be considered aFourier domain representation of the object or a frequency domainrepresentation of the object. A CGH may also be calculated by coherentray tracing or a point cloud technique, for example.

A CGH may be encoded on a spatial light modulator, “SLM”, arranged tomodulate the amplitude and/or phase of incident light. Light modulationmay be achieved using electrically-addressable liquid crystals,optically-addressable liquid crystals or micro-mirrors, for example.

The SLM may comprise a plurality of individually-addressable pixelswhich may also be referred to as cells or elements. The light modulationscheme may be binary, multilevel or continuous. Alternatively, thedevice may be continuous (i.e. is not comprised of pixels) and lightmodulation may therefore be continuous across the device. The SLM may bereflective meaning that modulated light is output from the SLM inreflection. The SLM may equally be transmissive meaning that modulatedlight is output from the SLM is transmission.

A holographic projector for imaging may be provided using the describedtechnology. Such projectors have found application in head-up displays,“HUD”, and head-mounted displays, “HMD”, including near-eye devices, forexample.

There is disclosed herein an improved holographic projection system.

SUMMARY

Aspects of the present disclosure are defined in the appendedindependent claims.

There is provided a holographic projector comprising a processingengine, spatial light modulator, light source and light-receivingsurface. The processing engine outputs a computer-generated diffractivepattern defining (or incorporating) a propagation distance to an imageplane. The spatial light modulator displays the computer-generateddiffractive pattern. The light source illuminates the spatial lightmodulator at an angle of incidence greater than zero. Thelight-receiving surface receives spatially-modulated light from thespatial light modulator. The light-receiving surface is substantiallyparallel to the spatial light modulator. The light-receiving surface isseparated from the spatial light modulator by the propagation distancedefined by the computer-generated diffractive pattern. Broad referenceis made herein to a light-receiving surface because the holographicreconstruction may be formed on any surface.

The spatial light modulator is illuminated with collimated light. Theperson skilled in the art of optics will be familiar with the concept of“normal incidence”. However, the present disclosure relates to so-called“off-axis illumination”. Specifically, the present disclosure relates tooff-axis illumination of a spatial light modulator displaying adiffractive pattern including a hologram. The term “off-axisillumination” is used herein to refer to cases in which the angle ofincidence of light on the spatial light modulator is non-zero or greaterthan zero. More specifically, the azimuth angle between a ray of theincident light and the normal to the plane of the spatial lightmodulator at the point of incidence is non-zero or greater than zero. Itmay therefore be said that the present disclosure relates to “non-normalincidence” of the spatial light modulator.

The light-receiving surface receives spatially-modulated light from thespatial light modulator. Light which is not diffracted by the spatiallight modulator (e.g. the so-called DC spot in the zero-order replayfield) follows an output path from the spatial light modulator which isdefined herein as the “output optical axis” of the spatial lightmodulator. The output optical axis is a straight-line between thespatial light modulator and light-receiving surface defining a generalpropagation direction of light from the spatial light modulator.

In projection, it is conventional to orientate the light-receivingsurface such that the normal to the light-receiving surface is parallelto the output optical axis. This conventionally forms the bestimage—particularly if the image is formed of image spots or pixels.However, the inventors have actually found that with holographicprojection this geometry gives rise to sub-optimal holographicreconstruction because there is an adverse effect on the size of theimage spots formed by the holographic process—particularly, the imagespots at the edge of the holographic replay field. The inventors hereindisclose that, in holographic projection using off-axis illumination ofthe spatial light modulator, the size of the holographic image spots inthe replay field can be reduced by tending towards parallelism betweenthe spatial light modulator and light-receiving surface. Smaller imagespots (c.f. image pixels) are advantageous in a display device.

The computer-generated diffractive pattern may be either (i) a Fourierhologram combined with a software lens function or (ii) a Fresnelhologram. In both cases, a physical lens (or any component having alensing effect) is not included in the propagation path from the spatiallight modulator to the light-receiving surface. It may be said that thepropagation of light from the spatial light modulator to thelight-receiving surface is a propagation only through free space. Inboth cases, it may be said that the distance from the spatial lightmodulator to the plane containing the light-receiving surface isentirely determined by the light modulation pattern displayed. Morespecifically, the perpendicular distance or shortest straight-linedistance between the spatial light modulator and light-receiving surfaceis entirely determined by the diffractive pattern. It may be consideredthat a lens component (or component providing a lensing effect onreceived light) is embedded or contained in the computer-generateddiffractive pattern and that lens component solely determines thedistance from the spatial light modulator to the light-receivingsurface.

Light modulation data representing the computer-generated diffractivepattern is provided to the spatial light modulator. The light modulationdata comprises an array of data values such as a 2D array of datavalues. The spatial light modulator may comprise a plurality of pixelsand each light modulation data value may be assigned to a correspondingpixel. In other words, each pixel of the spatial light modulator may beoperated at a light modulation level corresponding to a respective lightmodulation data value of the array of light modulation data values. Thedata values may be phase-delay values or amplitude-attenuation levels orboth.

In the Fourier case, the necessary lensing function is provided entirelyin software using lens data added to Fourier hologram data. Theso-called propagation distance from the spatial light modulator to thelight-receiving surface is determined—more specifically, solelydetermined or entirely determined—by the focusing power of the softwarelens emulated by the lens data. The propagation distance is equal to thefocal length of the software lens. The propagation distance is equal tothe Fourier path length which is, in turn, equal to the focal length ofthe software lens. The method may further comprise the lens performing afrequency-space transform of the Fourier transform hologram. In thiscase, the distance from the spatial light modulator to thelight-receiving surface is equal to the focal length of the lens.

In the Fresnel case, the propagation distance is a term in the Fresneltransform used to calculate the hologram. This term determines thedistance from the hologram plane to the image plane. That is, thedistance from the spatial light modulator to the focal plane where thelight-receiving surface should be positioned. It may therefore be saidthat the distance from the spatial light modulator to thelight-receiving surface is equal to the propagation distance, z, encodedin the Fresnel transform.

The angle may be less than 60 degrees, such as less than 45 degrees orless than 30 degrees. In embodiments described by way of example only,the angle is equal to or less than 20 degrees. In these cases, a morecompact system is provided. In practice, the angle may be optimised aspart of a larger system design.

The spatial light modulator may be a phase modulator and the lightmodulation data may comprise an array of phase-delay data values. In theFourier case, phase-delay data corresponding to a lens may be readilycalculated and combined with the hologram data of the hologram bywrapped addition which is not computationally demanding. Accordingly, aphase modulating scheme may be preferred.

The light-receiving surface may be diffuse. For example, thelight-receiving surface may be a diffuser. The light-receiving surfacemay be moving such as rotating or oscillating. Accordingly, no keystoneeffect or image stretching is observed in the replay field.

The spatial light modulator may be a liquid crystal on silicon spatiallight modulator and the spatial light modulator is illuminated withcoherent light. The light source may be a laser such as a laser diode.

The term “hologram” is used to refer to the recording which containsamplitude information or phase information, or some combination thereof,about the object. The term “holographic reconstruction” is used to referto the optical reconstruction of the object which is formed byilluminating the hologram. The term “replay plane” is used herein torefer to the plane in space where the holographic reconstruction isfully formed. The term “replay field” is used herein to refer to thesub-area of the replay plane which can receive spatially-modulated lightfrom the spatial light modulator. The terms “image”, “replay image” and“image region” refer to areas of the replay field illuminated by lightforming the holographic reconstruction. In embodiments, the “image” maycomprise discrete spots which may be referred to as “image pixels”.

The terms “encoding”, “writing” or “addressing” are used to describe theprocess of providing the plurality of pixels of the SLM with a respectplurality of control values which respectively determine the modulationlevel of each pixel. It may be said that the pixels of the SLM areconfigured to “display” a light modulation distribution in response toreceiving the plurality of control values. Thus, the SLM may be said to“display” a hologram.

It has been found that a holographic reconstruction of acceptablequality can be formed from a “hologram” containing only phaseinformation related to the original object. Such a holographic recordingmay be referred to as a phase-only hologram. Embodiments relate to aphase-only hologram but the present disclosure is equally applicable toamplitude-only holography.

The present disclosure is also equally applicable to forming aholographic reconstruction using amplitude and phase information relatedto the original object. In some embodiments, this is achieved by complexmodulation using a so-called fully complex hologram which contains bothamplitude and phase information related to the original object. Such ahologram may be referred to as a fully-complex hologram because thevalue (grey level) assigned to each pixel of the hologram has anamplitude and phase component. The value (grey level) assigned to eachpixel may be represented as a complex number having both amplitude andphase components. In some embodiments, a fully-complexcomputer-generated hologram is calculated.

Reference may be made to the phase value, phase component, phaseinformation or, simply, phase of pixels of the computer-generatedhologram or the spatial light modulator as shorthand for “phase-delay”.That is, any phase value described is, in fact, a number (e.g. in therange 0 to 2π) which represents the amount of phase retardation providedby that pixel. For example, a pixel of the spatial light modulatordescribed as having a phase value of π/2 will change the phase ofreceived light by π/2 radians. In some embodiments, each pixel of thespatial light modulator is operable in one of a plurality of possiblemodulation values (e.g. phase delay values). The term “grey level” maybe used to refer to the plurality of available modulation levels. Forexample, the term “grey level” may be used for convenience to refer tothe plurality of available phase levels in a phase-only modulator eventhough different phase levels do not provide different shades of grey.The term “grey level” may also be used for convenience to refer to theplurality of available complex modulation levels in a complex modulator.

The present disclosure refers to “lens data corresponding to a lenshaving a focal length”. This wording is used to reflect that the lensdata emulates or provides the functionality (i.e. focusing power) of alens such as a physical lens in the optical path. The lens data is alsoreferred to as a software lens. A software lens is used in preference toany physical lens.

Although different embodiments and groups of embodiments may bedisclosed separately in the detailed description which follows, anyfeature of any embodiment or group of embodiments may be combined withany other feature or combination of features of any embodiment or groupof embodiments. That is, all possible combinations and permutations offeatures disclosed in the present disclosure are envisaged.

BRIEF DESCRIPTION OF THE DRAWINGS

Specific embodiments are described by way of example only with referenceto the following figures:

FIG. 1 is a schematic showing a reflective SLM producing a holographicreconstruction on a screen;

FIG. 2A illustrates a first iteration of an example Gerchberg-Saxtontype algorithm;

FIG. 2B illustrates the second and subsequent iterations of the exampleGerchberg-Saxton type algorithm;

FIG. 2C illustrates alternative second and subsequent iterations of theexample Gerchberg-Saxton type algorithm;

FIG. 3 is a schematic of a reflective LCOS SLM;

FIG. 4A shows the basic optical set-up in accordance with the presentdisclosure;

FIG. 4B is a schematic corresponding to FIG. 4A;

FIG. 5 is a schematic of a replay field comprising a plurality of fieldpoints;

FIGS. 6A, 6B and 6C shows the image spots at the replay field inaccordance with embodiments;

FIGS. 7A, 7B and 7C shows the image spots at the replay field in analternative example;

FIGS. 8A, 8B and 8C shows the image spots at the replay field in afurther alternative example; and

FIGS. 9A, 9B and 9C shows the image spots at the replay field in a yetfurther alternative example.

The same reference numbers will be used throughout the drawings to referto the same or like parts.

DETAILED DESCRIPTION OF EMBODIMENTS

The present invention is not restricted to the embodiments described inthe following but extends to the full scope of the appended claims. Thatis, the present invention may be embodied in different forms and shouldnot be construed as limited to the described embodiments, which are setout for the purpose of illustration.

A structure described as being formed at an upper portion/lower portionof another structure or on/under the other structure should be construedas including a case where the structures contact each other and,moreover, a case where a third structure is disposed there between.

In describing a time relationship—for example, when the temporal orderof events is described as “after”, “subsequent”, “next”, “before” orsuchlike—the present disclosure should be taken to include continuousand non-continuous events unless otherwise specified. For example, thedescription should be taken to include a case which is not continuousunless wording such as “just”, “immediate” or “direct” is used.

Although the terms “first”, “second”, etc. may be used herein todescribe various elements, these elements are not be limited by theseterms. These terms are only used to distinguish one element fromanother. For example, a first element could be termed a second element,and, similarly, a second element could be termed a first element,without departing from the scope of the appended claims.

Features of different embodiments may be partially or overall coupled toor combined with each other, and may be variously inter-operated witheach other. Some embodiments may be carried out independently from eachother, or may be carried out together in co-dependent relationship.

Optical Configuration

FIG. 1 shows an example in which a computer-generated hologram isencoded on a single spatial light modulator. The computer-generatedhologram is a Fourier transform of the object for reconstruction. It maytherefore be said that the hologram is a Fourier domain or frequencydomain or spectral domain representation of the object. In thisembodiment, the spatial light modulator is a reflective liquid crystalon silicon, “LCOS”, device. The hologram is encoded on the spatial lightmodulator and a holographic reconstruction is formed at a replay field,for example, a light receiving surface such as a screen or diffuser.

A light source 110, for example a laser or laser diode, is disposed toilluminate the SLM 140 via a collimating lens 111. The collimating lenscauses a generally planar wave-front of light to be incident on the SLM.In FIG. 1 , the direction of the wave-front is off-normal (e.g. two orthree degrees away from being truly orthogonal to the plane of thetransparent layer). However, in other embodiments, the generally planarwave-front is provided at normal incidence and a beam splitterarrangement is used to separate the input and output optical paths. Inthe example shown in FIG. 1 , the arrangement is such that light fromthe light source is reflected off a mirrored rear surface of the SLM andinteracts with a light-modulating layer to form an exit wave-front 112.The exit wave-front 112 is applied to optics including a Fouriertransform lens 120, having its focus at a screen 125. More specifically,the Fourier transform lens 120 receives a beam of modulated light fromthe SLM 140 and performs a frequency-space transformation to produce aholographic reconstruction at the screen 125.

Notably, in this type of holography, each pixel of the hologramcontributes to the whole reconstruction. There is not a one-to-onecorrelation between specific points (or image pixels) on the replayfield and specific light-modulating elements (or hologram pixels). Inother words, modulated light exiting the light-modulating layer isdistributed across the replay field.

The position of the holographic reconstruction in space is determined bythe dioptric (focusing) power of the Fourier transform lens. In FIG. 1 ,the Fourier transform lens is a physical lens. That is, the Fouriertransform lens is an optical Fourier transform lens and the Fouriertransform is performed optically. Any lens can act as a Fouriertransform lens but the performance of the lens will limit the accuracyof the Fourier transform it performs. The skilled person understands howto use a lens to perform an optical Fourier transform.

Hologram Calculation

In some embodiments, the computer-generated hologram is a Fouriertransform hologram, or simply a Fourier hologram or Fourier-basedhologram, in which an image is reconstructed in the far field byutilising the Fourier transforming properties of a positive lens. TheFourier hologram is calculated by Fourier transforming the desired lightfield in the replay plane back to the lens plane. Computer-generatedFourier holograms may be calculated using Fourier transforms.

A Fourier transform hologram may be calculated using an algorithm suchas the Gerchberg-Saxton algorithm. Furthermore, the Gerchberg-Saxtonalgorithm may be used to calculate a hologram in the Fourier domain(i.e. a Fourier transform hologram) from amplitude-only information inthe spatial domain (such as a photograph). The phase information relatedto the object is effectively “retrieved” from the amplitude-onlyinformation in the spatial domain. In some embodiments, acomputer-generated hologram is calculated from amplitude-onlyinformation using the Gerchberg-Saxton algorithm or a variation thereof.

The Gerchberg Saxton algorithm considers the situation when intensitycross-sections of a light beam, I_(A)(x, y) and I_(B)(x, y), in theplanes A and B respectively, are known and I_(A)(x, y) and I_(B)(x, y)are related by a single Fourier transform. With the given intensitycross-sections, an approximation to the phase distribution in the planesA and B, ψ_(A)(x, y) and ψ_(B)(x, y) respectively, is found. TheGerchberg-Saxton algorithm finds solutions to this problem by followingan iterative process. More specifically, the Gerchberg-Saxton algorithmiteratively applies spatial and spectral constraints while repeatedlytransferring a data set (amplitude and phase), representative ofI_(A)(x, y) and I_(B)(x, y), between the spatial domain and the Fourier(spectral or frequency) domain. The corresponding computer-generatedhologram in the spectral domain is obtained through at least oneiteration of the algorithm. The algorithm is convergent and arranged toproduce a hologram representing an input image. The hologram may be anamplitude-only hologram, a phase-only hologram or a fully complexhologram.

In some embodiments, a phase-only hologram is calculated using analgorithm based on the Gerchberg-Saxton algorithm such as described inBritish patent 2,498,170 or 2,501,112 which are hereby incorporated intheir entirety by reference. However, embodiments disclosed hereindescribe calculating a phase-only hologram by way of example only. Inthese embodiments, the Gerchberg-Saxton algorithm retrieves the phaseinformation ψ[u, v] of the Fourier transform of the data set which givesrise to a known amplitude information T[x, y], wherein the amplitudeinformation T[x, y] is representative of a target image (e.g. aphotograph). Since the magnitude and phase are intrinsically combined inthe Fourier transform, the transformed magnitude and phase containuseful information about the accuracy of the calculated data set. Thus,the algorithm may be used iteratively with feedback on both theamplitude and the phase information. However, in these embodiments, onlythe phase information ψ[u, v] is used as the hologram to form aholographic representative of the target image at an image plane. Thehologram is a data set (e.g. 2D array) of phase values.

In other embodiments, an algorithm based on the Gerchberg-Saxtonalgorithm is used to calculate a fully-complex hologram. A fully-complexhologram is a hologram having a magnitude component and a phasecomponent. The hologram is a data set (e.g. 2D array) comprising anarray of complex data values wherein each complex data value comprises amagnitude component and a phase component.

In some embodiments, the algorithm processes complex data and theFourier transforms are complex Fourier transforms. Complex data may beconsidered as comprising (i) a real component and an imaginary componentor (ii) a magnitude component and a phase component. In someembodiments, the two components of the complex data are processeddifferently at various stages of the algorithm.

FIG. 2A illustrates the first iteration of an algorithm in accordancewith some embodiments for calculating a phase-only hologram. The inputto the algorithm is an input image 210 comprising a 2D array of pixelsor data values, wherein each pixel or data value is a magnitude, oramplitude, value. That is, each pixel or data value of the input image210 does not have a phase component. The input image 210 may thereforebe considered a magnitude-only or amplitude-only or intensity-onlydistribution. An example of such an input image 210 is a photograph orone frame of video comprising a temporal sequence of frames. The firstiteration of the algorithm starts with a data forming step 202Acomprising assigning a random phase value to each pixel of the inputimage, using a random phase distribution (or random phase seed) 230, toform a starting complex data set wherein each data element of the setcomprising magnitude and phase. It may be said that the starting complexdata set is representative of the input image in the spatial domain.

First processing block 250 receives the starting complex data set andperforms a complex Fourier transform to form a Fourier transformedcomplex data set. Second processing block 253 receives the Fouriertransformed complex data set and outputs a hologram 280A. In someembodiments, the hologram 280A is a phase-only hologram. In theseembodiments, second processing block 253 quantises each phase value andsets each amplitude value to unity in order to form hologram 280A. Eachphase value is quantised in accordance with the phase-levels which maybe represented on the pixels of the spatial light modulator which willbe used to “display” the phase-only hologram. For example, if each pixelof the spatial light modulator provides 256 different phase levels, eachphase value of the hologram is quantised into one phase level of the 256possible phase levels. Hologram 280A is a phase-only Fourier hologramwhich is representative of an input image. In other embodiments, thehologram 280A is a fully complex hologram comprising an array of complexdata values (each including an amplitude component and a phasecomponent) derived from the received Fourier transformed complex dataset. In some embodiments, second processing block 253 constrains eachcomplex data value to one of a plurality of allowable complex modulationlevels to form hologram 280A. The step of constraining may includesetting each complex data value to the nearest allowable complexmodulation level in the complex plane. It may be said that hologram 280Ais representative of the input image in the spectral or Fourier orfrequency domain. In some embodiments, the algorithm stops at thispoint.

However, in other embodiments, the algorithm continues as represented bythe dotted arrow in FIG. 2A. In other words, the steps which follow thedotted arrow in FIG. 2A are optional (i.e. not essential to allembodiments).

Third processing block 256 receives the modified complex data set fromthe second processing block 253 and performs an inverse Fouriertransform to form an inverse Fourier transformed complex data set. Itmay be said that the inverse Fourier transformed complex data set isrepresentative of the input image in the spatial domain.

Fourth processing block 259 receives the inverse Fourier transformedcomplex data set and extracts the distribution of magnitude values 211Aand the distribution of phase values 213A. Optionally, the fourthprocessing block 259 assesses the distribution of magnitude values 211A.Specifically, the fourth processing block 259 may compare thedistribution of magnitude values 211A of the inverse Fourier transformedcomplex data set with the input image 510 which is itself, of course, adistribution of magnitude values. If the difference between thedistribution of magnitude values 211A and the input image 210 issufficiently small, the fourth processing block 259 may determine thatthe hologram 280A is acceptable. That is, if the difference between thedistribution of magnitude values 211A and the input image 210 issufficiently small, the fourth processing block 259 may determine thatthe hologram 280A is a sufficiently-accurate representative of the inputimage 210. In some embodiments, the distribution of phase values 213A ofthe inverse Fourier transformed complex data set is ignored for thepurpose of the comparison. It will be appreciated that any number ofdifferent methods for comparing the distribution of magnitude values211A and the input image 210 may be employed and the present disclosureis not limited to any particular method. In some embodiments, a meansquare difference is calculated and if the mean square difference isless than a threshold value, the hologram 280A is deemed acceptable. Ifthe fourth processing block 259 determines that the hologram 280A is notacceptable, a further iteration of the algorithm may performed. However,this comparison step is not essential and in other embodiments, thenumber of iterations of the algorithm performed is predetermined orpreset or user-defined.

FIG. 2B represents a second iteration of the algorithm and any furtheriterations of the algorithm. The distribution of phase values 213A ofthe preceding iteration is fed-back through the processing blocks of thealgorithm. The distribution of magnitude values 211A is rejected infavour of the distribution of magnitude values of the input image 210.In the first iteration, the data forming step 202A formed the firstcomplex data set by combining distribution of magnitude values of theinput image 210 with a random phase distribution 230. However, in thesecond and subsequent iterations, the data forming step 202B comprisesforming a complex data set by combining (i) the distribution of phasevalues 213A from the previous iteration of the algorithm with (ii) thedistribution of magnitude values of the input image 210.

The complex data set formed by the data forming step 202B of FIG. 2B isthen processed in the same way described with reference to FIG. 2A toform second iteration hologram 280B. The explanation of the process isnot therefore repeated here. The algorithm may stop when the seconditeration hologram 280B has been calculated. However, any number offurther iterations of the algorithm may be performed. It will beunderstood that the third processing block 256 is only required if thefourth processing block 259 is required or a further iteration isrequired. The output hologram 280B generally gets better with eachiteration. However, in practice, a point is usually reached at which nomeasurable improvement is observed or the positive benefit of performinga further iteration is out-weighted by the negative effect of additionalprocessing time. Hence, the algorithm is described as iterative andconvergent.

FIG. 2C represents an alternative embodiment of the second andsubsequent iterations. The distribution of phase values 213A of thepreceding iteration is fed-back through the processing blocks of thealgorithm. The distribution of magnitude values 211A is rejected infavour of an alternative distribution of magnitude values. In thisalternative embodiment, the alternative distribution of magnitude valuesis derived from the distribution of magnitude values 211 of the previousiteration. Specifically, processing block 258 subtracts the distributionof magnitude values of the input image 210 from the distribution ofmagnitude values 211 of the previous iteration, scales that differenceby a gain factor α and subtracts the scaled difference from the inputimage 210. This is expressed mathematically by the following equations,wherein the subscript text and numbers indicate the iteration number:R _(n+1) [x,y]=F′{exp(iψ _(n) [u,v])}ψ_(n) [u,v]=∠F{η·exp(i∠R _(n) [x,y])}η=T[x,y]−α(|R _(n) [x,y]|−T[x,y])where:F′ is the inverse Fourier transform;F is the forward Fourier transform;R[x, y] is the complex data set output by the third processing block256;T[x, y] is the input or target image;∠ is the phase component;ψ is the phase-only hologram 280B;η is the new distribution of magnitude values 211B; andα is the gain factor.

The gain factor α may be fixed or variable. In some embodiments, thegain factor α is determined based on the size and rate of the incomingtarget image data. In some embodiments, the gain factor α is dependenton the iteration number. In some embodiments, the gain factor α issolely function of the iteration number.

The embodiment of FIG. 2C is the same as that of FIG. 2A and FIG. 2B inall other respects. It may be said that the phase-only hologram ψ(u, v)comprises a phase distribution in the frequency or Fourier domain.

In accordance with the present disclosure, the hologram includes datarepresentative of a lens as well as data representing the object. Thephysical Fourier transform lens 120 shown in FIG. 1 is not present. Itis known in the field of computer-generated hologram how to calculateholographic data representative of a lens. The holographic datarepresentative of a lens may be referred to as a software lens. Forexample, a phase-only holographic lens may be formed by calculating thephase delay caused by each point of the lens owing to its refractiveindex and spatially-variant optical path length. For example, theoptical path length at the centre of a convex lens is greater than theoptical path length at the edges of the lens. An amplitude-onlyholographic lens may be formed by a Fresnel zone plate. It is also knownin the art of computer-generated hologram how to combine holographicdata representative of a lens with holographic data representative ofthe object so that a Fourier transform can be performed without the needfor a physical Fourier lens. In some embodiments, lensing data iscombined with the holographic data by simple addition such as simplevector addition. In further embodiments, the hologram may includegrating data—that is, data arranged to perform the function of a gratingsuch as beam steering. Again, it is known in the field ofcomputer-generated holography how to calculate such holographic data andcombine it with holographic data representative of the object. Forexample, a phase-only holographic grating may be formed by modelling thephase delay caused by each point on the surface of a blazed grating. Anamplitude-only holographic grating may be simply superimposed on anamplitude-only hologram representative of an object to provide angularsteering of an amplitude-only hologram.

In some embodiments, there is provided a real-time engine arranged toreceive image data and calculate holograms in real-time using thealgorithm. In some embodiments, the image data is a video comprising asequence of image frames. In other embodiments, the holograms arepre-calculated, stored in computer memory and recalled as needed fordisplay on a SLM. That is, in some embodiments, there is provided arepository of predetermined holograms.

Embodiments relate to Fourier holography and Gerchberg-Saxton typealgorithms by way of example only. The present disclosure is equallyapplicable to Fresnel holography.

Light Modulation

A spatial light modulator may be used to display the computer-generatedhologram. If the hologram is a phase-only hologram, a spatial lightmodulator which modulates phase is required. If the hologram is afully-complex hologram, a spatial light modulator which modulates phaseand amplitude may be used or a first spatial light modulator whichmodulates phase and a second spatial light modulator which modulatesamplitude may be used.

In some embodiments, the light-modulating elements (i.e. the pixels) ofthe spatial light modulator are cells containing liquid crystal. Thatis, in some embodiments, the spatial light modulator is a liquid crystaldevice in which the optically-active component is the liquid crystal.Each liquid crystal cell is configured to selectively-provide aplurality of light modulation levels. That is, each liquid crystal cellis configured at any one time to operate at one light modulation levelselected from a plurality of possible light modulation levels. Eachliquid crystal cell is dynamically-reconfigurable to a different lightmodulation level from the plurality of light modulation levels. In someembodiments, the spatial light modulator is a reflective liquid crystalon silicon (LCOS) spatial light modulator but the present disclosure isnot restricted to this type of spatial light modulator.

A LCOS device provides a dense array of light modulating elements, orpixels, within a small aperture (e.g. a few centimeters in width). Thepixels are typically approximately 10 microns or less which results in adiffraction angle of a few degrees meaning that the optical system canbe compact. It is easier to adequately illuminate the small aperture ofa LCOS SLM than it is the larger aperture of other liquid crystaldevices. An LCOS device is typically reflective which means that thecircuitry which drives the pixels of a LCOS SLM can be buried under thereflective surface. The results in a higher aperture ratio. In otherwords, the pixels are closely packed meaning there is very little deadspace between the pixels. This is advantageous because it reduces theoptical noise in the replay field. A LCOS SLM uses a silicon backplanewhich has the advantage that the pixels are optically flat. This isparticularly important for a phase modulating device.

A suitable LCOS SLM is described below, by way of example only, withreference to FIG. 3 . An LCOS device is formed using a single crystalsilicon substrate 302. It has a 2D array of square planar aluminiumelectrodes 301, spaced apart by a gap 301 a, arranged on the uppersurface of the substrate. Each of the electrodes 301 can be addressedvia circuitry 302 a buried in the substrate 302. Each of the electrodesforms a respective planar mirror. An alignment layer 303 is disposed onthe array of electrodes, and a liquid crystal layer 304 is disposed onthe alignment layer 303. A second alignment layer 305 is disposed on theplanar transparent layer 306, e.g. of glass. A single transparentelectrode 307 e.g. of ITO is disposed between the transparent layer 306and the second alignment layer 305.

Each of the square electrodes 301 defines, together with the overlyingregion of the transparent electrode 307 and the intervening liquidcrystal material, a controllable phase-modulating element 308, oftenreferred to as a pixel. The effective pixel area, or fill factor, is thepercentage of the total pixel which is optically active, taking intoaccount the space between pixels 301 a. By control of the voltageapplied to each electrode 301 with respect to the transparent electrode307, the properties of the liquid crystal material of the respectivephase modulating element may be varied, thereby to provide a variabledelay to light incident thereon. The effect is to provide phase-onlymodulation to the wavefront, i.e. no amplitude effect occurs.

The described LCOS SLM outputs spatially modulated light in reflection.Reflective LCOS SLMs have the advantage that the signal lines, gatelines and transistors are below the mirrored surface, which results inhigh fill factors (typically greater than 90%) and high resolutions.Another advantage of using a reflective LCOS spatial light modulator isthat the liquid crystal layer can be half the thickness than would benecessary if a transmissive device were used. This greatly improves theswitching speed of the liquid crystal (a key advantage for theprojection of moving video images). However, the teachings of thepresent disclosure may equally be implemented using a transmissive LCOSSLM.

Relative Tilt of Spatial Light Modulator and Light-Receiving Surface

FIG. 4A shows three light channels. Each light channel comprises a lightsource, spatial light modulator and light-receiving surface. Each lightchannel provides a holographic reconstruction in one colour.Accordingly, a composite colour holographic reconstruction may beprovided by using a plurality of single colour channels such as red,green and blue channels and overlapping the single colour replay fieldsat the replay plane. The hologram of each channel is tailored to thecolour content of that channel. FIG. 4A shows three light channels byway of example only. The three channels are substantially parallel andmay share a common spatial light modulator—for example, a subset ofpixels of the common spatial light modulator may be allocated to eachrespective colour channel—or each channel may have its own spatial lightmodulator. The three corresponding replay fields may be coincident atthe replay plane. The teachings of the present disclosure are equallyapplicable to a holographic projector comprising one light channel orany number of light channels. For simplicity, reference is made in thefollowing to the components of just one of the light channels.

FIG. 4A shows a light source 401A illuminating a corresponding liquidcrystal on silicon spatial light modulator 403A. The light is incidenton the spatial light modulator 403A at an angle greater than zero to thenormal of the spatial light modulator 403A. The spatial light modulator403A is planar and reflective so the spatially-modulated light is outputat the same angle to the normal of the spatial light modulator 403A. Thespatially modulated light is received by the light-receiving surface405A.

The same is shown schematically in FIG. 4B. A light source 401Billuminates a spatial light modulator 403B at an angle of θ to thenormal of the spatial light modulator 403B. A light-receiving surface405B receives the spatially-modulated light from the spatial lightmodulator 403B at an angle of a to the normal of the light-receivingsurface 405B. In embodiments, the spatial light modulator 403B andlight-receiving surface 405B are parallel. That is, θ is equal to a. Inexamples shown, θ is 20 degrees but θ may have any non-zero value.

Each pixel of the spatial light modulator 403A/403B displays arespective light modulation level of light modulation data. The lightmodulation data comprises hologram data corresponding to an image forprojection. The light modulation data also comprises lens datacorresponding to a lens having an optical power. The lens has a focallength. The holographic reconstruction is formed at the plane of thelight-receiving surface 405A/405B owing to the focusing power of thelens data. As explained above, the lens data may be termed a “softwarelens” and is a mathematical function representative of a physical lens.The software lens provides the same functionality— namely, focusingpower—as a physical optical lens of the same dioptric power. Thesoftware lens may be an array of phase-delay values corresponding to theshape of the corresponding optical component. A holographicreconstruction of the image is formed on the light-receiving surface405A/405B. The lens may perform a mathematical transform—such as aFourier transform—of the hologram. It will be understood that a Fouriertransform is a frequency-space transform. In embodiments using a Fouriertransform hologram, it may be said that the hologram is a frequencydomain representation of the image for projection, the holographicprojection is a spatial domain representation of the image and thesoftware lens performs a frequency-space transform of the hologram.

Again, the present disclosure relates to a specific case in which θ isnon-zero (in other words, greater than zero) and the inventors haveobserved that, in this specific case, the size of the image spots in theholographic reconstruction of a hologram displayed on the spatial lightmodulator can be reduced by tending towards parallelism between thespatial light modulator and light-receiving surface. Ray tracingsoftware has been used to verify this finding. The inventors found that,in the off-axis scheme, the size of the image spots isdiffraction-limited when the spatial light modulator and light-receivingsurface are parallel. A sample of results are provided herein by way ofexample but the technical effect achieved has been observed over thefull scope of the claims. In particular, the technical effect observedhas been found at all possible angles of incidence on the spatial lightmodulator greater than zero—that is, the off-axis angle of incidence onthe spatial light modulator may be any angle greater than zero and lessthan 90 degrees.

FIG. 5 is a schematic of a holographic replay field 501 and is providedfor the purpose of better understanding FIGS. 6 to 9 . The replay field501 in FIG. 5 shows nine so-called field points, labelled FP1 to FP9,which are points in the replay field. Ray tracing has been used todetermine the size and shape of the image spot at each of field pointsFP1 to FP9 for a range of tilt angles.

FIGS. 6 to 9 show nine image spots corresponding to the nine fieldpoints, FP1 to FP9, of FIG. 5 . The image spots corresponding to thefield points are display in ascending numerical order from left toright. That is, the image spot at FP1 is shown to the extreme left andthe image spot at FP9 is shown to the extreme right.

Each of FIGS. 6 to 9 show three sets of image spots in three respectiverows. The top set of image spots (FIGS. 6A, 7A, 8A and 9A) are formedusing blue light having a wavelength of 450 nm, the middle set of imagespots (FIGS. 6B, 7B, 8B and 9B) are formed using green light having awavelength of 520 nm and the bottom set of image spots (FIGS. 6C, 7C, 8Cand 9C) are formed using red light having a wavelength of 650 nm. Thesolid circle or dot shown for each image spot is the correspondingdiffraction limit.

The results shown in FIGS. 6 to 9 are achieved in the cases shown inTable 1. The angles shown in Table 1 are angles in degrees to thenormal. The relative angle is the angle of the SLM relative to thelight-receiving surface and amounts to the difference between θ and α.

TABLE 1 angles used to achieve the results of FIGS. 6 to 9 and Tables 2to 5. FIGURE θ α Relative angle 6 20 20  0 7  5 10  5 8 20 40 20 9 −20 40 −20 

The size of the image spot at each field point has been measured andthese measurements are shown below in micrometres. The “relative angle”in Tables 2 to 5 is the angle, in degrees, of the light-receivingsurface relative to the spatial light modulator.

For the avoidance of any doubt, all image spots in a holographic replayfield are formed at the same time from the same computer-generateddiffractive pattern. For example, the nine image spots at the nine fieldpoints FP1 to FP9 are formed at the same time. This is in contrast tobeam scanning systems in which each image is formed bit by bit.

TABLE 2 Image spot size measurements corresponding to FIG. 6(diffraction limit is 4.287 μm) FIGURE FP Relative angle Spot size 6A 10 3.446 6A 2 0 3.509 6A 3 0 5.057 6A 4 0 3.509 6A 5 0 2.002 6A 6 0 5.1226A 7 0 5.122 6A 8 0 2.072 6A 9 0 2.072 6B 1 0 3.228 6B 2 0 3.321 6B 3 05.103 6B 4 0 3.321 6B 5 0 1.583 6B 6 0 5.19  6B 7 0 5.19  6B 8 0 1.8146B 9 0 1.814 6C 1 0 2.987 6C 2 0 3.15  6C 3 0 5.366 6C 4 0 3.15  6C 5 01.886 6C 6 0 5.526 6C 7 0 5.526 6C 8 0 2.221 6C 9 0 2.221

TABLE 3 Image spot size measurements corresponding to FIG. 7(diffraction limit is 4.141 μm). FIGURE FP Relative angle Spot size 7A 15 0.657 7A 2 5 0.776 7A 3 5 14.241 7A 4 5 0.776 7A 5 5 13.401 7A 6 514.407 7A 7 5 14.07 7A 8 5 13.42 7A 9 5 13.42 7B 1 5 0.462 7B 2 5 0.627B 3 5 16.172 7B 4 5 0.62 7B 5 5 15.676 7B 6 5 16.387 7B 7 5 16.387 7B 85 15.693 7B 9 5 15.693 7C 1 5 0.442 7C 2 5 0.488 7C 3 5 19.914 7C 4 50.488 7C 5 5 19.75 7C 6 5 20.238 7C 7 5 20.238 7C 8 5 19.762 7C 9 519.762

TABLE 4 Image spot size measurements corresponding to FIG. 8(diffraction limit is 4.027 μm). FIGURE FP Relative angle Spot size 8A 120 3.556 8A 2 20 3.991 8A 3 20 66.671 8A 4 20 3.981 8A 5 20 61.004 8A 620 62.36 8A 7 20 62.36 8A 8 20 61.43 8A 9 20 61.43 8B 1 20 3.376 8B 2 203.941 8B 3 20 70.89 8B 4 20 3.941 8B 5 20 70.798 8B 6 20 71.813 8B 7 2071.813 8B 8 20 71.274 8B 9 20 71.274 8C 1 20 3.167 8C 2 20 4.071 8C 3 2088.182 8C 4 20 4.071 8C 5 20 89.031 8C 6 20 89.62 8C 7 20 89.62 8C 8 2089.455 8C 9 20 89.455

TABLE 5 Image spot size measurements corresponding to FIG. 9(diffraction limit is 4.284 μm). FIGURE FP Relative angle Spot size 9A 1−20 3.565 9A 2 −20 3.981 9A 3 −20 55.512 9A 4 −20 3.981 9A 5 −20 61.3679A 6 −20 56.039 9A 7 −20 56.039 9A 8 −20 62.029 9A 9 −20 62.029 9B 1 −203.376 9B 2 −20 3.941 9B 3 −20 64.82 9B 4 −20 3.941 9B 5 −20 70.132 9B 6−20 65.416 9B 7 −20 65.416 9B 8 −20 70.982 9B 9 −20 70.982 9C 1 −203.167 9C 2 −20 4.071 9C 3 −20 82.011 9C 4 −20 4.071 9C 5 −20 86.528 9C 6−20 82.733 9C 7 −20 82.733 9C 8 −20 87.797 9C 9 −20 87.797

The results in Tables 2 to 5 show that, for off-axis illumination,smaller image spots are formed if the spatial light modulator andlight-receiving surface are parallel (i.e. the relative angle is zero).Smaller image spots are preferable because they provide higherresolution in the holographic replay field.

Additional Features

Embodiments refer to an electrically-activated LCOS spatial lightmodulator by way of example only. The teachings of the presentdisclosure may equally be implemented on any spatial light modulatorcapable of displaying a computer-generated hologram in accordance withthe present disclosure such as any electrically-activated SLMs,optically-activated SLM, digital micromirror device ormicroelectromechanical device, for example.

In some embodiments, the light source is a laser such as a laser diode.In some embodiments, the light receiving surface is a diffuser surfaceor screen such as a diffuser. The holographic projection system of thepresent disclosure may be used to provide an improved head-up display(HUD) or head-mounted display. In some embodiments, there is provided avehicle comprising the holographic projection system installed in thevehicle to provide a HUD. The vehicle may be an automotive vehicle suchas a car, truck, van, lorry, motorcycle, train, airplane, boat, or ship.

The quality of the holographic reconstruction may be affect by theso-called zero order problem which is a consequence of the diffractivenature of using a pixelated spatial light modulator. Such zero-orderlight can be regarded as “noise” and includes for example specularlyreflected light, and other unwanted light from the SLM.

In the example of Fourier holography, this “noise” is focused at thefocal point of the Fourier lens leading to a bright spot at the centreof the holographic reconstruction. The zero-order light may be simplyblocked out however this would mean replacing the bright spot with adark spot. Some embodiments include an angularly selective filter toremove only the collimated rays of the zero order. Embodiments alsoinclude the method of managing the zero-order described in Europeanpatent 2,030,072, which is hereby incorporated in its entirety byreference.

In some embodiments, the size (number of pixels in each direction) ofthe hologram is equal to the size of the spatial light modulator so thatthe hologram fills the spatial light modulator. That is, the hologramuses all the pixels of the spatial light modulator. In otherembodiments, the size of the hologram is less than the size of thespatial light modulator. In some of these other embodiments, part of thehologram (that is, a continuous subset of the pixels of the hologram) isrepeated in the unused pixels. This technique may be referred to as“tiling” wherein the surface area of the spatial light modulator isdivided up into a number of “tiles”, each of which represents at least asubset of the hologram. Each tile is therefore of a smaller size thanthe spatial light modulator.

In some embodiments, the technique of “tiling” is implemented toincrease image quality. Specifically, some embodiments implement thetechnique of tiling to minimise the size of the image pixels whilstmaximising the amount of signal content going into the holographicreconstruction.

In some embodiments, the holographic pattern written to the spatiallight modulator comprises at least one whole tile (that is, the completehologram) and at least one fraction of a tile (that is, a continuoussubset of pixels of the hologram).

The holographic reconstruction is created within the zeroth diffractionorder of the overall window defined by the spatial light modulator. Itis preferred that the first and subsequent orders are displaced farenough so as not to overlap with the image and so that they may beblocked using a spatial filter.

In embodiments, the holographic reconstruction is colour. In examplesdisclosed herein, three different colour light sources and threecorresponding SLMs are used to provide composite colour. These examplesmay be referred to as spatially-separated colour, “SSC”. In a variationencompassed by the present disclosure, the different holograms for eachcolour are displayed on different area of the same SLM and thencombining to form the composite colour image. However, the skilledperson will understand that at least some of the devices and methods ofthe present disclosure are equally applicable to other methods ofproviding composite colour holographic images.

One of these methods is known as Frame Sequential Colour, “FSC”. In anexample FSC system, three lasers are used (red, green and blue) and eachlaser is fired in succession at a single SLM to produce each frame ofthe video. The colours are cycled (red, green, blue, red, green, blue,etc.) at a fast enough rate such that a human viewer sees apolychromatic image from a combination of the images formed by threelasers. Each hologram is therefore colour specific. For example, in avideo at 25 frames per second, the first frame would be produced byfiring the red laser for 1/75th of a second, then the green laser wouldbe fired for 1/75th of a second, and finally the blue laser would befired for 1/75th of a second. The next frame is then produced, startingwith the red laser, and so on.

An advantage of FSC method is that the whole SLM is used for eachcolour. This means that the quality of the three colour images producedwill not be compromised because all pixels of the SLM are used for eachof the colour images. However, a disadvantage of the FSC method is thatthe overall image produced will not be as bright as a correspondingimage produced by the SSC method by a factor of about 3, because eachlaser is only used for a third of the time. This drawback couldpotentially be addressed by overdriving the lasers, or by using morepowerful lasers, but this would require more power to be used, wouldinvolve higher costs and would make the system less compact.

An advantage of the SSC method is that the image is brighter due to allthree lasers being fired at the same time. However, if due to spacelimitations it is required to use only one SLM, the surface area of theSLM can be divided into three parts, acting in effect as three separateSLMs. The drawback of this is that the quality of each single-colourimage is decreased, due to the decrease of SLM surface area availablefor each monochromatic image. The quality of the polychromatic image istherefore decreased accordingly. The decrease of SLM surface areaavailable means that fewer pixels on the SLM can be used, thus reducingthe quality of the image. The quality of the image is reduced becauseits resolution is reduced. Embodiments utilise the improved SSCtechnique disclosed in British patent 2,496,108 which is herebyincorporated in its entirety by reference.

Examples describe illuminating the SLM with visible light but theskilled person will understand that the light sources and SLM mayequally be used to direct infrared or ultraviolet light, for example, asdisclosed herein. For example, the skilled person will be aware oftechniques for converting infrared and ultraviolet light into visiblelight for the purpose of providing the information to a user. Forexample, the present disclosure extends to using phosphors and/orquantum dot technology for this purpose.

Some embodiments describe 2D holographic reconstructions by way ofexample only. In other embodiments, the holographic reconstruction is a3D holographic reconstruction. That is, in some embodiments, eachcomputer-generated hologram forms a 3D holographic reconstruction.

The methods and processes described herein may be embodied on acomputer-readable medium. The term “computer-readable medium” includes amedium arranged to store data temporarily or permanently such asrandom-access memory (RAM), read-only memory (ROM), buffer memory, flashmemory, and cache memory. The term “computer-readable medium” shall alsobe taken to include any medium, or combination of multiple media, thatis capable of storing instructions for execution by a machine such thatthe instructions, when executed by one or more processors, cause themachine to perform any one or more of the methodologies describedherein, in whole or in part.

The term “computer-readable medium” also encompasses cloud-based storagesystems. The term “computer-readable medium” includes, but is notlimited to, one or more tangible and non-transitory data repositories(e.g., data volumes) in the example form of a solid-state memory chip,an optical disc, a magnetic disc, or any suitable combination thereof.In some example embodiments, the instructions for execution may becommunicated by a carrier medium. Examples of such a carrier mediuminclude a transient medium (e.g., a propagating signal that communicatesinstructions).

It will be apparent to those skilled in the art that variousmodifications and variations can be made without departing from thescope of the appended claims. The present disclosure covers allmodifications and variations within the scope of the appended claims andtheir equivalents.

The invention claimed is:
 1. A holographic projector comprising: aprocessing engine arranged to output a computer-generated diffractivepattern defining a propagation distance to an image plane; a planarspatial light modulator arranged to display the computer-generateddiffractive pattern; a light source arranged to illuminate the spatiallight modulator at an angle of incidence (θ) greater than zero relativeto a surface normal of the spatial light modulator, wherein the spatiallight modulator is arranged to output spatially-modulated light on apropagation path having an axis so as to form a holographicreconstruction of the computer-generated diffractive pattern on theimage plane; a screen arranged at the image plane to receivespatially-modulated light from the spatial light modulator, wherein thescreen is arranged substantially parallel to the spatial light modulatorsuch that the axis of the propagation path of the spatially-modulatedlight received from the spatial light modulator by the screen is at anangle of incidence (α) relative to a surface normal of the screen thatis substantially equal to the angle of incidence (θ) of light from thelight source on the spatial light modulator, and wherein the screen isseparated from the spatial light modulator by the propagation distancealong the axis of the propagation path defined by the computer-generateddiffractive pattern, wherein the computer-generated diffractive patternis one selected from the group comprising: the sum of a Fourier hologramand lens function corresponding to a lens having a focal length equal tothe propagation distance; and a Fresnel hologram.
 2. A holographicprojector as claimed in claim 1, wherein the angle of incidence (θ) oflight on the spatial light modulator is less than 60 degrees.
 3. Aholographic projector as claimed in claim 2, wherein the angle ofincidence (θ) of light on the spatial light modulator is greater than orequal to 2 degrees.
 4. A holographic projector as claimed in claim 1,wherein the spatial light modulator is a phase modulator and thecomputer-generated diffractive pattern is a distribution of phase-delayvalues.
 5. A holographic projector as claimed in claim 1 wherein theprocessing engine is arranged to calculate the computer-generateddiffractive pattern.
 6. A holographic projector as claimed in claim 1wherein the screen is a diffuser.
 7. A head-up display comprising aholographic projector as claimed in claim
 1. 8. A holographic projectoras claimed in claim 1, wherein the angle of incidence (θ) of light onthe spatial light modulator is greater than or equal to 2 degrees.
 9. Amethod of holographic projection, the method comprising: providing acomputer-generated diffractive pattern defining a propagation distanceto an image plane, wherein the computer-generated diffractive pattern is(i) the combination of a Fourier hologram and lens functioncorresponding to a lens having a focal length equal to the propagationdistance or (ii) a Fresnel hologram; displaying the computer-generateddiffractive pattern on a planar spatial light modulator; illuminatingthe spatial light modulator at an angle of incidence (θ) greater thanzero relative to a surface normal of the spatial light modulator;outputting spatially-modulated light on a propagation path having anaxis so as to form a holographic reconstruction of thecomputer-generated diffractive pattern on the image plane; receivingspatially-modulated light from the spatial light modulator on a screenat the image plane, wherein the screen is substantially parallel to thespatial light modulator such that the axis of the propagation path ofthe spatially-modulated light received from the spatial light modulatorby the screen is at an angle of incidence (α) relative to a surfacenormal of the screen substantially equal to the angle of incidence (θ)of light from the light source on the spatial light modulator, whereinthe screen is separated from the spatial light modulator by thepropagation distance along the axis of the propagation path defined bythe computer-generated diffractive pattern.
 10. A method of holographicprojection as claimed in claim 9, the method further comprisingcalculating the computer-generated diffractive pattern using a Fouriertransform between a spatial domain and a frequency domain.
 11. A methodof holographic projection as claimed in claim 9, wherein the angle ofincidence (θ) of light on the spatial light modulator is less than 60degrees.
 12. A method of holographic projection as claimed in claim 11,wherein the angle of incidence (θ) of light on the spatial lightmodulator is greater than or equal to 2 degrees.
 13. A method ofholographic projection as claimed in claim 9, wherein the angle ofincidence (θ) of light on the spatial light modulator is greater than orequal to 2 degrees.